The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


Lexical::Types - Extend the semantics of typed lexicals.


Version 0.16


    { package Str; }

     package My::Types::Str;

     sub new { bless { }, shift }

    use Lexical::Types as => sub { 'My::Types::' . $_[0] => 'new' };

    my Str $x; # $x is now a My::Types::Str object

     package My::Types::Int;

     sub TYPEDSCALAR { bless { }, shift }

    use Lexical::Types;

    use constant Int => 'My::Types::Int';

    my Int $y; # $y is now a My::Types::Int object


This pragma allows you to hook the execution of typed lexicals declarations (my Str $x) by calling a configurable method in a configurable package at each run. In particular, it can be used to automatically tie or bless typed lexicals whenever they are initialized.

Remind that for perl to be able to parse my Str $x, you need :

  • either the Str package to be defined ;

  • or for Str to be a constant sub returning a valid defined package.

so make sure you follow one of those two strategies to define your types.

This pragma is not implemented with a source filter.



    use Lexical::Types;
    use Lexical::Types as => $prefix;
    use Lexical::Types as => sub { ... }; # = $mangler

Magically called when use Lexical::Types is encountered. All the occurences of my Str $x in the current lexical scope will be changed to call at each run a given method in a given package. The method and package are determined by the parameter 'as' :

  • If it's left unspecified, the TYPEDSCALAR method in the Str package will be called.

        use Lexical::Types;
        my Str $x; # calls Str->TYPEDSCALAR
  • If a plain scalar $prefix is passed as the value, the TYPEDSCALAR method in the ${prefix}::Str package will be used.

        use Lexical::Types as => 'My::'; # or "as => 'My'"
        my Str $x; # calls My::Str->TYPEDSCALAR
  • If the value given is a code reference $mangler, it will be called at compile-time with arguments 'Str' and 'TYPEDSCALAR' and is expected to return :

    • either an empty list, in which case the current typed lexical definition will be skipped (thus it won't be altered to trigger a run-time hook) ;

          use Lexical::Types as => sub {
           return $_[0] =~ /Str/ ? @_ : ()
          my Str $y; # calls Str->TYPEDSCALAR
          my Int $x; # nothing special
    • or the desired package and method name, in that order (if any of those is undef, the default value will be used instead).

          use Lexical::Types as => sub { 'My', 'new_' . lc($_[0]) };
          my Str $x; # the coderef indicates to call My->new_str

    Note that if the type is a constant, $_[0] will be set to the value of constant and not to its name.

        use Lexical::Types as => sub { $_[0] => 'new' };
        use constant Str => 'MyStr';
        my Str $x; # calls MyStr->new

    This means in particular that you can't both use constant types and redirect several types to different methods of the same package, because then you can't distinguish between the original types with $_[0].


    no Lexical::Types;

Magically called when no Lexical::Types is encountered. Turns the pragma off.


The initializer method receives an alias to the pad slot of the initialized lexical in $_[1] and the original type name in $_[2]. You can either edit $_[1] in place, in which case you should return an empty list, or return a new scalar that will be copied into the pad slot.

    use Lexical::Types as => 'My';

    my Str $x;


    sub My::Str::TYPEDSCALAR {
     # $_[1] is an alias to $x, and $_[2] is 'Str'


You can integrate Lexical::Types in your module so that using it will provide types to your users without asking them to load either Lexical::Types or the type classes manually.

    package MyTypes;

    BEGIN { require Lexical::Types; }

    sub import {
     eval 'package Str; package Int'; # The types you want to support
      as => sub { __PACKAGE__, 'new_' . lc($_[0]) }

    sub unimport {

    sub new_str { ... }

    sub new_int { ... }

If you prefer to use constants rather than creating empty packages, you can replace the previous example with something like this :

    package MyTypes;

    BEGIN { require Lexical::Types; }

    sub import {
     my $pkg = caller;
     for (qw<Str Int>) {
      my $type = __PACKAGE__ . '::' . $_;
      no strict 'refs';
      no warnings 'redefine';
      *{$pkg.'::'.$_} = eval "sub () { '$type' }";
      as => sub { $_[0] => 'new' }

    sub unimport {

    package MyTypes::Str;

    sub new { ... }

    package MyTypes::Int;

    sub new { ... }



True iff the module could have been built with thread-safety features enabled.


True iff this module could have been built with fork-safety features enabled. This will always be true except on Windows where it's false for perl 5.10.0 and below .


Using this pragma will cause a slight global slowdown of any subsequent compilation phase that happens anywere in your code - even outside of the scope of use of use Lexical::Types - which may become noticeable if you rely heavily on numerous calls to eval STRING.

The restrictions on the type (being either a defined package name or a constant) apply even if you use the 'as' option to redirect to another package, and are unlikely to find a workaround as this happens deep inside the lexer - far from the reach of an extension.

Only one mangler or prefix can be in use at the same time in a given scope.

Typed lexicals declarations that appear in code eval'd during the global destruction phase of a spawned thread or pseudo-fork (the processes used internally for the fork emulation on Windows) are ignored.

The implementation was tweaked to work around several limitations of vanilla perl pragmas : it's thread safe, and doesn't suffer from a perl 5.8.x-5.10.0 bug that causes all pragmas to propagate into required scopes.

With 5.8 perls, the pragma does not propagate into eval STRING. This is due to a shortcoming in the way perl handles the hints hash, which is addressed in perl 5.10.


perl 5.8.4.

A C compiler. This module may happen to build with a C++ compiler as well, but don't rely on it, as no guarantee is made in this regard.

XSLoader (standard since perl 5.6.0).





Vincent Pit, <perl at>,

You can contact me by mail or on (vincent).


Please report any bugs or feature requests to bug-lexical-types at, or through the web interface at I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.


You can find documentation for this module with the perldoc command.

    perldoc Lexical::Types


Inspired by Ricardo Signes.

Thanks Florian Ragwitz for suggesting the use of constants for types.


Copyright 2009,2010,2011,2012,2013,2014,2015,2017 Vincent Pit, all rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.